Dissolved organic carbon as a driver of seasonal and multiyear phytoplankton assembly oscillations in a subtropical monomictic lake

Abstract

Phytoplankton assembly dynamics in lakes are highly sensitive to variability in climate drivers and resulting physicochemical changes in lake water columns. As climate change increases the frequency of major precipitation events and droughts, many lakes experience increased inputs of colored dissolved organic carbon (CDOC) and nutrients. How these CDOC-related changes in resources, transparency, and thermal stability affect phytoplankton assemblages, succession, and resilience is understudied, particularly in subtropical lakes. Here, we used time series, multivariate, and trait-based functional redundancy analyses to elucidate the roles of phytoplankton in ecosystem resilience and determine potential drivers of assemblage shifts in a subtropical monomictic lake with fluctuating CDOC inputs (Lake Annie, Highlands County, Florida, USA). We found that phytoplankton assemblages and successional patterns differed between two dark-water states (late 2005–mid-2007, late 2012–2019) bracketing a clear-water state (mid-2007–late 2012), caused by shifting CDOC and nutrient concentrations associated with oscillating groundwater levels. Diatoms (Bacillariophyta), which were dominant during the two dark-water states, nearly disappeared and were replaced by synurophytes during the clear-water state. Assemblages had greater interannual consistency in the dark-water states, while mean functional redundancy decreased in the clear-water state. Seasonal phytoplankton successional changes were also more pronounced and synchronized with seasonal hydrologic shifts in the dark-water states. Multiyear assemblage shifts occurred more quickly in clear-to-dark than dark-to-clear state transitions, suggesting phytoplankton in dark-water states may be more resistant to state transitions or even contribute to dark-water state resilience via feedback loops.